The ironic fact is that Nick's unfinished sentence is, in a sense finished; it finishes the theme of F. Scott Fitzgerald's novel, The Great Gatsby. For, it underlines the theme of the American Dream as illusionary.

Nick's ruminations about Gatsby's legacy and his valiant, though futile attempts, to regain his lost love, are as romantic an idea as those of the Arthurian legends. Thus, they speak to the necessity in man for myth--something to believe in beyond himself, something that makes his life worth living, the struggle meaningful. In the end, therefore, Fitzgerald expresses clearly the illusion of the American Dream, but he does not dispute the necessity of the Americans to dream.

This is just one more instance in the novel demonstrating Nick's unfailing hope for the future. While his pursuit of Daisy ran into many dead ends, first being sent off to the war, his return home and the realization that he didn't have the status to pursue her, his finding out that she was married to the boor Tom, then everything that happens in the novel, through it all, he still has this idealized view of Daisy herself and of the opportunities for them to be together.

The fact that the sentence isn't finished mimics the entire concept in Nick's mind that it is never finished, his quest will never be completed successfully or in failure so he will continue to reach out and one fine day...